Choice of 17 O correction affects clumped isotope (Δ47 ) values of CO2 measured with mass spectrometry

Abstract

Rationale: The clumped isotope composition of CO₂ (Δ₄₇ ) derived from carbonate is widely used as a paleotemperature proxy with broad applications in geoscience. Its accuracy is, however, limited by inter-laboratory discrepancies of reference materials and disagreement among carbonate geothermometer calibrations. Here we show how the correction for the abundance of ¹⁷ O influences these discrepancies.

Methods: We used CO₂ -H₂ O equilibration at known temperatures and phosphoric acid digested carbonates to generate CO₂ samples with a wide range in ¹³ C and ¹⁸ O compositions. All samples were purified using an offline vacuum line. We used a Thermo MAT 253 isotope ratio mass spectrometer with a Faraday collector array for m/z 44-49, to measure δ¹³ C, δ¹⁸ O, δ⁴⁷ , and Δ₄₇ values. Data were compiled using a traditional correction ('Santrock') for the ¹⁷ O interference in m/z 45 as well as a more recently proposed correction ('Brand') for ¹⁷ O interference. Two reference frames using CO₂ with distinct ¹³ C compositions were constructed to simulate an inter-laboratory comparison.

Results: The traditional Santrock ¹⁷ O correction leads to a simulated Δ₄₇ inter-laboratory comparison offset of 0.06 ‰, and a 0.1 ‰ Δ₄₇ range in CO₂ -H₂ O 23°C equilibrations that is dependent on the ¹³ C composition. The more recent Brand ¹⁷ O correction removes these discrepancies. The traditional ¹⁷ O correction yields distinct temperature-Δ₄₇ calibration curves for synthetic carbonates precipitated using different methods to degas CO₂ , while the more recent ¹⁷ O correction collapses all calibration data onto a single curve.

Conclusions: The ¹⁷ O correction strategy employed by CO₂ and carbonate clumped-isotope researchers can have a large effect on the accuracy of Δ₄₇ values. Use of the traditional ¹⁷ O correction may have caused errors in published studies as large as 0.1 ‰ and may account for Δ₄₇ differences among laboratories and disagreement among previously published carbonate clumped isotope thermometry calibrations. Copyright © 2016 John Wiley & Sons, Ltd.